A piston fitted with O-ring seals is arranged to slide in the body bush assembly by means of the operating mechanism. The bush is drilled with a series of holes connecting by an annulus with each of the valve ports in the valve body. The arrangement of the O-ring grooves and the holes is such that different ports are interconnected by movement of the piston.

On the ZNCN/562 range of Piston Valves the main air supply can usually be connected to any of the five threaded ports. But in the case of valves utilizing an air return mechanism (stroke 45) the main air supply must be connected to the 'inlet' port as the integral pilot supply is taken from this port. They are also of the 'supply sealed mid - position' type which means that when the piston is in the mid-position, the supply port only is isolated and sealed. Valve units of the ZNCN/564 series are externally and dimensionally the same as the ZNCN/562 types, the difference being that the ZNCN/564 fully sealed mid - position. In the mid - position all the ports are sealed and isolated from one another. Piston Valve versions are fully sealed in the mid-position in other words, when the piston is in the mid-position, all five ports are sealed and isolated from one another.